1. Field of the Invention
The present invention relates to a process for carrying out multiphase reactions in a tube reactor, in particular for preparing xcex1,xcex2-unsaturated aldehydes by aldol condensation of aldehydes.
2. Discussion of the Related Art
Because of their reactivity, unsaturated aldehydes are starting materials for the preparation of many organic compounds. Their selective hydrogenation results in the corresponding saturated aldehydes which are also the basis of many syntheses. The oxidation of aldehydes leads to carboxylic acids which are utilized industrially. The hydrogenation of aldehydes leads to saturated alcohols which are used for preparing plasticizers and detergents.
The aldol reaction of n-butyraldehyde with simultaneous elimination of water to give 2-ethylhexenal is carried out worldwide on a large scale since the hydrogenation product, 2-ethylhexanol, is widely used as a plasticizer alcohol. A base dissolved in water is customarily employed as catalyst. Typically, use is made of aqueous sodium hydroxide solution having a NaOH content in the percentage range. The reaction is frequently carried out in a temperature range of 80-150xc2x0 C., a pressure below 5 bar and a phase ratio of organic phase to catalyst phase of 1:20 (Hydrocarbon Processing, October 1980, Section 2, pages 93-102). This reaction can be performed, for example, in a stirred vessel (DE 19 06 850, DE 927 626), in a packed column operated in countercurrent (G. Dxc3xcmbgen, D. Neubauer, Chemie-Ing.-Techn., 41, 974 (1969)), or in a flow tube (GB 761 203). All these processes give 2-ethylhexenal in a selectivity of up to 98% at conversions of 98.5%. A disadvantage is that at relatively high temperatures part of the n-butyraldehyde used is irreversibly lost as a result of the Cannizzaro reaction. The butyric acid formed in the Cannizzaro reaction neutralizes the basic catalyst. Part of the catalyst solution containing a high loading of organic material therefore has to be discharged continually and replaced by fresh catalyst.
Valeraldehyde can be reacted similarly to give 2-propylheptenal. The aldol condensation of the C5-aldehydes can be carried out in stirred reactors which are equipped with internal heat exchangers to remove the heat. This reaction procedure is described, for example, in WO 93/20034 and is, because of the moving parts, susceptible to mechanical problems and in addition, is complicated in terms of construction and maintenance because of the heat exchangers built into the reactor.
In the case of aldehydes having more than 6 carbon atoms, the reaction rate is still lower because of the low solubility of the aldehydes in the aqueous catalyst phase and is therefore often no longer economical. It is likewise difficult to carry out the aldol condensation of branched aldehydes such as 3-methylbutanal.
The condensation of aldehydes can also be carried out in the homogeneous phase, e.g. using amines as a catalyst. These processes have the disadvantages that more by-products are formed and the catalyst has to be separated from the product mixture. For this reason, industrial aldol condensations are preferably carried out as multiphase reactions, in particular as two-phase reactions.
For the purposes of the following, two-phase reactions are reactions which occur with the participation of two fluid phases which are immiscible or only partially miscible. In the aldol condensation of aldehydes, two liquid phases which are immiscible or have a miscibility gap are present. At the beginning of the reaction, the two phases consist of the starting material and the catalyst solution, while after the reaction is complete they consist of the product and the catalyst phases.
In every two-phase reaction, the problem of mass transfer has to be overcome. The starting materials have to be transported into the catalyst phase and the products may have to be transported back. Since transport processes are frequently slower than the actual reaction, such reactions are determined by the rate of mass transfer, so the reaction is referred to as a mass-transfer-limited reaction.
In order to obtain industrially acceptable space-time yields in a multiphase reaction, particularly in one in which the phases are virtually insoluble in one another, the materials have to be brought into very intimate contact with one another. It is necessary to generate a mass transfer area a, between the phases which is as large as possible. On the other hand, the phases have to be able to be separated easily again after the reaction is complete. Excessive mixing can lead to difficulties here, since emulsions can be formed.
Apart from a high mass transfer area as, a very high mass transfer coefficient k1 should be achieved in all multiphase reactions. Overall, the KLA value, i.e. the product of k1 and as in the mass transfer equation is:
j=k1as(C*xe2x88x92C)
where
j [mol/s]: the molar flow of reacting component through the phase interface (e.g. entry of aldehyde into the catalyst phase),
k1 [m/s]: mass transfer coefficient,
as [m2]: phase interface area in the reactor (e.g. aldehyde in the catalyst phase),
C* [mol/m3]: maximum solubility of the starting material in the second phase and
C [mol/m3]: actual concentration of the starting material which in turn is coupled to the reaction rate,
should be a maximum.
A further problem in multiphase reactions is the removal of heat in the case of an exothermic reaction. If the reaction rate is increased by improving the mass transfer, it is naturally also necessary to remove more heat which could lead to an undesirable temperature increase and even cause a runaway reaction.
For this reason, a two-phase aldol condensation is frequently carried out in a stirred reactor. However, in a stirred reactor, one has to accept the continual back-mixing which reduces the effective concentration of the reactants. This leads to a lowering of the space-time yield, which in turn has to be compensated for by an increased reaction volume.
Alternatively, the two-phase reaction could also be carried out in a flow tube. Here, however, there is the danger of the phases separations and the reaction rate decreasing to an excessive extent. Furthermore, the above-discussed problems of heat removal have to be taken into account.
It is therefore an object of the present invention to provide a process for carrying out multiphase reactions, which process is particularly suitable for carrying out aldol condensations.
In technical terms, the novel process should meet the following demands made of multiphase processes:
generation of high and stable mass transfer between the participating phases;
simplicity of implementation, if possible using customary industrial apparatuses;
simple and reliable heat removal;
high operating safety; and
simple and reliable scale-up.
In respect of the intended preparation of (xcex1,xcex2-unsaturated aldehydes by aldol condensation, the following specific requirements are added:
high selectivity, avoidance of, in particular, high-boiling by-products;
suppression of the Cannizzaro reaction, therefore no or only small catalyst discharge;
high space-time yield, small reactors; and
high product purity.
The present invention accordingly provides a process for the catalytic aldol condensation of aldehydes by means of a multiphase reaction in a tube reactor, wherein the catalyst is present in a continuous phase and at least one aldehyde is present in a dispersed phase and the loading factor B of the reactor is equal to or greater than 0.8.